Thinking about... low stake Fenton Family headcanons. Those kind of things that have no wider world-context when it comes to fic ideas but really speak to their family dynamic.
Like, one of my favourites is that Danny doesn't avoid swearing because it's a kid's show from 2004; he avoids swearing because when he and Jazz were little, Maddie and Jack caught them repeating what what they'd heard from the lab. So they promised their children that if they could make it the next decade or so, until they got their drivers licenses, without swearing in front of them, Maddie and Jack would buy Jazz and Danny each their own car! They assumed the kids would forget before they had to follow through, but Jazz did not. And once she was sixteen, she traded the inability to swear for a car.
Danny can fly. He doesn't technically need to get his driver's license. But by this point he's committed, and won't swear at all - he doesn't want to risk slipping up in front of his parents, and losing his chance at a car!
Another one I really love is that up until he was about ten, Danny spent most of his time at the public pool. Maddie had him and Jazz in baby swim lessons, but Danny loved the water, so she kept him in it. And Danny was an amazing swimmer - he was incredibly fast and by the time he was seven, could dive well enough that there was barely a splash when he hit the water. Danny thrived in the water, and with some guidance, absolutely could have become an olympic level swimmer.
Unfortunately, when he was 10 or so, he mentioned to his parents that the chlorine in the pool hurt his eyes so Jack tried to replace the pool filter with an ecto-based one, which went very badly. And the family was banned from the public pool. They set up a blow-up pool in the backyard for Danny during the summer, but it wasn't the same.
Fortunately, he turned 14 and gained the ability to fly, so he doesn't miss it too much anymore, but Jack does still feel bad.
And of course, the last quality one - none of the Fenton family can cook. Maddie and Jack can bake extremely well, but baking and cooking are different. They like the experiment too much with their cooking, and even without ectoplasm, it usually leaves their food inedible. Jazz can make simple things, like macaroni, but whenever she's tried something more complicated it hasn't gone to plan. She's too much of a perfectionist to risk that.
Danny, meanwhile, has burned soup before and will burn it again. But he's still eaten it. Sam and Tucker have joked about how Danny's tastebuds must've died in the portal too, because there's no way a normal person would eat the kind of things he does, but he's always been like that! He's far too ready to eat almost anything.
It was particularly stressful for his family when Danny was a baby, because it seemed like for the first four years of his life, his sole goal was to eat every single battery he could find.
